1. Field of the Invention
The present invention relates generally to an apparatus and process for transferring liquids, especially liquids, such as molten sulfur, which are difficult to handle, from one location or container to another by the use of a pressurized gas.
2. Background Discussion
Sulfur is the fifteenth most common terrestrial element and is of great commercial importance. According to published statistics, about 9.8 million tons of sulfur were produced in the United States in 1986, about 49 percent of which was used--principally in the form of sulfuric acid--for industrial purposes, including the production of petrochemicals, plastics and fibers. Another 25 percent of the produced sulfur was used for inorganics and pigments, and about 12 percent for non-chemical purposes, such as plating. The remaining 14 percent or so of the produced sulfur was used, indirectly or directly, in agriculture--about 9 percent as sulfuric acid for the production of phosphate fertilizers and the remaining 5 percent for application to crops and the like.
Unlike most elements, sulfur is produced by both "voluntary" and "involuntary" means. In "voluntary" production, sulfur is intentionally mined and produced from naturally-occurring ores or deposits, with such production being entirely discretionary on the part of the sulfur producers. By contrast, in "involuntary" production, sulfur is produced as a necessary by-product of other processes, or from the manufacture of other products. Consequently, involuntary sulfur production depends upon the market for the other processes or products and not upon the demand for sulfur.
With regard to the involuntary production of sulfur, large quantities of elemental sulfur are, for example, obtained from unwanted hydrogen sulfide and/or sulfur removed from natural gas, crude oil, and geothermal fluids during the production, processing, or use of these fluids. Natural gas, for example, typically contains between about 15 and 30 percent of hydrogen sulfide which must be removed, to meet pollution standards, before or during use of the gas. Moreover, in addition to usually containing some hydrogen sulfide, crude oils typically contain between about 0.1 and 2.8 percent of elemental sulfur, with some "sour" oils having over a 3 percent sulfur content; most of this sulfur must be removed from the crude oil during its refining.
More than half the sulfur presently produced in the United States is produced involuntarily. For example, of the approximately 9.8 million tons of sulfur produced in the United States in 1986, only about 4.0 million tons were "voluntarily" produced, mainly by the Frash process. Of the remaining 5.8 million tons of "involuntary" sulfur, about 2.24 million tons were reportedly produced as a by-product of cleaning natural gas.
This high percentage of involuntarily-produced sulfur can and does cause substantial upsets in the sulfur market. In the early 1970s, for example, the Mideast oil embargoes forced a greatly increased reliance on higher sulfur-content, "sour" crude oils from other regions of the world. As a consequence, involuntary sulfur producers (principally in the oil and gas industry) accumulated huge surpluses of sulfur, thereby causing a worldwide sulfur surplus and a substantial decrease in the market value of sulfur. The curtailing of voluntary sulfur production and the resumed usage of lower sulfur-content oil has since reduced these huge sulfur surpluses of the early 1970s; nevertheless, sulfur surpluses still, from time to time, occur. More recent surpluses of sulfur have, for example, been caused by such factors as the diminished demand for sulfur for producing phosphate fertilizers (due to improved crop strains and the over-productions of food in many countries) and the still-increasing use of sour oil from Texas, Mexico, and Venezuela.
Mainly because of such sulfur surpluses, new and/or expanded uses for sulfur have been sought in order to stabilize the sulfur market. Most of the new or proposed new, uses for sulfur are for structural materials, principally: (i) sulfur-asphalt compositions for road building, (ii) rigid sulfur foams for thermal insulation, and (iii) sulfur-based concrete for special applications in which the properties of conventional, portland cement-based concretes are inadequate.
Regarding the combining of sulfur with asphalt--with which the present invention is indirectly concerned--it has been well known for over a century that sulfur can improve the properties of asphalt compositions. For example, the addition of sulfur to asphalt can result in the increased stability of asphalt pavements (macadam), and in reduced pavement rutting, washboarding, and deflections. However, only in recent years have the necessary techniques been developed to the extent that sulfur and asphalt can be combined in a practical manner.
Sulfur can be incorporated into asphalt for paving in either of two principal ways, each of which has a different purpose. One such way is to incorporate molten sulfur in a hot mix; the other way is to produce an asphalt/sulfur emulsion. By adding about 13 percent of sulfur in the hot mix asphalt process, most or all of the generally costly (and increasingly scarce) rock aggregate, which would normally be used in the paving material, can be replaced with much less costly, and more readily available, sand. Although the added sulfur increases the fluidity of the hot mix, when the mix cools the sulfur solidifies and contributes to the mechanical stability of the mixture.
In the sulfur/asphalt emulsion process, molten sulfur replaces some of the asphalt oil binder, which is usually more costly than sulfur. Such so-called "sulfur-extended asphalts" typically contain 30 to 50 percent of sulfur which may be emulsified into the asphalt by a special mixer.
Other processes for using sulfur as a replacement for asphalt in a plasticized sulfur composition have reportedly been developed for the U.S. Federal Highway Administration and tested by the U.S. Bureau of Mines. These plasticized sulfur compositions contain substantial amounts of such plasticizers as dicyclopentadiene. However, the high cost of the plasticizers is presently impeding significant development of the material.
Along with the interest of the sulfur industry in developing new uses and markets for sulfur, a Strategic Highway Research Program (SHRP) has recently been established in the United States to provide carefully targeted research toward improving highway materials and pavement performance so as to preserve the trillion dollar investment in United States highways. One specially targeted area of research for the $150 million, 5-year study program by SHRP is asphalt, since of the slightly over 2 million miles of paved highways in the United States, nearly 1.9 million miles consist, at least in part, of asphaltic materials. In this regard, about 30-35 million tons of asphalt paving material are reportedly used each year just in the State of California.
One of the problems associated with the use of molten sulfur for such purposes as compounding asphalt paving materials is that sulfur has a fairly high melting point of 115.2.degree. C. (about 240.degree. F.). Relatively costly systems are, therefore, presently required for storing and transferring molten sulfur, which is commonly delivered to a road-building site in liquid form by tank trucks typically containing about 23 to 24 tons (about 3200 gallons) of sulfur. Moreover, such molten sulfur handling and transferring systems are required to be mobile to the extent they can be advanced along a roadway with other equipment as the sulfur/asphalt pavement composition is applied. To keep the sulfur in its molten state, such systems typically require a steam-jacketed tank for storing the molten sulfur, a boiler for generating steam for the steam jacket, and a pump and piping for continuously recirculating molten sulfur through the discharge pipe used to deliver the sulfur to apparatus in which the sulfur is to be mixed when needed. Some type of molten sulfur metering or weighing equipment is additionally required so that the proper amount of molten sulfur can be mixed with asphalt and aggregate or sand to make the sulfur/asphalt paving material.
The relatively high cost of such molten sulfur handling and transfer systems--the estimated cost for each such system is between about $50,000 and about $100,000--tends to make it difficult to generate great interest in the use by paving contractors of sulfur as an asphalt pavement component, particularly since there is not presently a large surplus of sulfur and its cost is not particularly low.